1. Introduction
This invention relates to a developer for a bilayer photoresist film. More particularly, the invention relates to a developer capable of selectively developing the bottom layer of a bilayer resist film without significant attack on the top layer of the film.
2. Description of the Prior Art
Etch resistant masks are commonly fabricated in the manufacture of integrated circuits and other microminiature electronic devices. In a typical fabrication process, a radiation sensitive layer of a photoresist is coated onto a substrate and exposed in an image pattern to activating radiation such as visible, ultraviolet, x-ray, nuclear or electron beam radiation. The radiated portions of the resist layer undergo a chemical change which converts the resist material either to a more soluble (positive resist) or less soluble(negative resist) form than the non-radiated areas. A developer is used to preferentially remove the more soluble areas, which in a positive resist, are the radiated areas, and in a negative resist, are the non-radiated areas. Following development, the substrate may be subjected to selective processing steps through the openings in the resist layer or mask, for example by etching or deposition.
The size of a semiconductor device is a factor in the ultimate speed of an integrated circuit as well as in the initial and operational cost of the device. Therefore, efforts have been made to reduce the size of individua components and to increase the packing density of integrated components. Size reduction is limited by the accuracy with which an etch resistant mask can be fabricated and positioned. Because certain processing steps such as electroplating, reactive ion etching and lift off require a relatively thick mask, the limiting factors in a multilevel fabrication process often are mask resolution and aspect ratio (thickness of mask/minimum practical line width at that thickness) which can be achieved during fabrication of thick etch resistant masks. Resolution and aspect ratio are limited in part by the choice of resist material and activating radiation and in part by the type and resolution of the exposure system.
Positive working photoresists comprising an o-quinone diazide sensitizer in a phenol-formaldehyde resin are in common use. A typical resist is composed of a base soluble polymer such as a phenol-formaldehyde novolak resin and a photosensitive compound such as a naphthoquinone-(1,2)-diazide sulfonic acid ester sensitizer. Such resists and sensitizers from which they are formulated are described, for example, in U.S. Pat. Nos. 3,046,118; 3,046,121; 3,106,465; 3,201,239; 3,666,473; and 4,007,047; all incorporated herein by reference. O-quinone diazide sensitized phenol-formaldehyde resists have high sensitivity and submicron resolution when the resist layer thickness is sufficiently small that the diffraction and absorption effects do not limit image resolution. Thick resist layers of this type (greater than one micron in thickness) have a low aspect ratio and a much reduced resolution due to optical diffraction and absorption effects.
Another positive resist known to the art contains a partially aqueous soluble imidized acrylic polymer in a non-aqueous solvent. Such a resist is disclosed in U.S. Pat. No. 4,524,121 incorporated herein by reference. The aqueous soluble, imidized acrylic polymers, known in the art as "polyglutarimides", can be dissolved in a non-reactive non-aqueous solvent to form a positive resist system that can be deposited as an adherent film on a substrate. Such films are capable of high image resolution.
When a photoresist such as a polyglutarimide or an o-quinone diazide sensitized phenol-formaldehyde is exposed by optical projection, depth of field will limit image resolution and aspect ratio unless the photoresist layer is thin. While it is possible to avoid a depth of field problem by forming an image with a narrow photon beam and computer controlling it to directly write a pattern onto a resist layer, this approach is not practical for high volume production because it involves precision manipulation of an optical lens system which is slow and impractical. Exposure by contact printing also avoids depth of field problems, but it involves other disadvantages. Contact printing tends to scratch masks so that contact masks have a short life. Proximity printing rather than contact printing extends the life of the mask but diffraction effects are worse. Diffraction effects may be reduced by reducing the wavelength of the exposure light, but this type of improvement is ultimately limited by the light sensitivity and absorption characteristics of the resist. Because of these problems, it has not been practical to fabricate high resolution thick masks from either diazo sensitized phenol-formaldehyde or polyglutarimide positive photoresists.
U.S. Pat. No. 4,211,834, incorporated herein by reference, proposes a method for fabricating a thick, etch-resistant image with high aspect ratio and resolution while avoiding or reducing diffraction effects. This patent utilizes a bilayer resist approach and is based in part upon the recognition that o-quinone diazide sensitized phenol-formaldehyde resists are opaque below 3,000 A while other positive working resists are sensitive to radiation below 3,000 A. Utilizing the difference in spectral response, a first thick resist layer may be applied over a substrate and coated with a thin layer of a second resist. The first or bottom resist layer would be one responsive to activating radiation below 3,000 A, such as within the deep ultra violet range, while the second or top resist layer would be one responsive above 3,000 A. In accordance with the patent, polymethyl methacrylate is used as the bottom resist layer. In accordance with the teachings of others, it has also been found that the polyglutarimides, referenced above, may be used as a bottom resist layer because they are also responsive to activating radiation below 3,000 A.
As aforesaid, a bilayer resist is formulated with a relatively thick bottom resist layer and a relatively thin top resist layer. The top resist layer is imaged and developed, using activating radiation having a wavelength in excess of 3,000 A. Development of the top resist layer results in a relief image which acts as a conformable mask over the bottom resist layer. The bottom resist layer may then be flooded with radiation of a wavelength lower than 3,000 A. Because the top resist layer of the diazide phenol-formaldehyde resist is opaque to radiation below 3,000 A, it is not exposed to this radiation and prevents exposure of the bottom resist layer beneath it. However, where the top resist layer has been removed by development, the bottom resist layer is exposed to this activating radiation and following exposure, can be developed to yield a relief image conforming to the relief image in the top resist layer.
The advantage to the bilayer resist film is that the diazide phenol-formaldehyde top resist layer may be made suitably thin to avoid resolution limiting effects arising from light diffraction or depth of field while the bottom resist layer may be made sufficiently thick to perform desired processing steps which require thick resist layers and high aspect ratio without losing the high resolution achieved by the thin top resist layer.
In theory, the bilayer resist should solve many of the problems described above. This is not the case because in practice, the material used to formulate the bottom layer of the bilayer film is a polymethyl methacrylate. The use of polymethyl methacrylate results in interlayer mixing which creates processing problems. In addition, the polymethyl methacrylate layer has a thermal flow point of about 120.degree. C. and therefore has a tendency to flow during processing. Further, the polymethyl methacrylates do not possess adequate plasma resistance. Finally, the polymethyl methacrylates are developed in organic solvents which require special venting and waste treatment procedures.
Polyglutarimide resists have processing properties superior to those of polymethyl methacrylate. However, both the polyglutarimide resists and the diazo phenol formaldehyde resists are developed in aqueous alkaline solutions of tetramethyl ammonium hydroxide or sodium hydroxide. In practice, it has been found that exposure and development of the top diazo phenol-formaldehyde resist layer followed by exposure and development of the bottom polyglutarimide resist layer results in excessive attack by the developer on the top resist layer resulting in substantial erosion of this layer, often to a point where the top resist layer is completely removed during development of the bottom resist layer. Since the top resist layer is a conformable mask for the bottom resist layer, the loss of the top resist layer during development results in loss of plasma resistance. In addition, the diazo phenol-formaldehyde type resists are known to have excellent plasma etch resistance. This etch resistance is superior to the etch resistance of the polyglutarimides. Consequently, loss of the diazo phenol formaldehyde top resist layer results in an overall decrease in the protection provided the substrate from plasma etching.